With the recent energy and environmental problems, fuel cells are of great interest as a highly efficient and nonpolluting generating system that can produce clean energy from hydrogen without emitting greenhouse gases such as CO2. Currently, extensive studies and research are carried out for establishing use of the fuel cells in fixed facilities such as houses and business offices and mobile facilities such as automobiles.
The fuel cells are categorized by the types of electrolytes employed, such as, alkaline electrolyte type, solid polymer electrolyte type, phosphoric acid type, molten carbonate type and solid electrolyte type. In the solid polymer electrolyte fuel cells and the phosphoric acid fuel cells, protons are responsible for the charge transfer; therefore, these cells are also known as proton fuel cells.
Fuels for use in the above fuel cells include hydrocarbon fuels such as natural gas, LP gas, city gas, alcohols, gasoline, kerosene and gas oil.
The hydrocarbon fuel is converted to hydrogen and CO gases by reaction such as moisture reforming or partial oxidation, and the CO gas is eliminated to obtain the hydrogen gas. The hydrogen is supplied to anodes and dissociated by a metal catalyst into protons (hydrogen ions) and electrons. The electrons do their jobs as passing through circuits to cathodes, whilst the protons (hydrogen ions) are diffused through the electrolyte membrane to reach the cathodes. At the cathodes, the electrons, the hydrogen ions, and the oxygen supplied at the cathodes react to produce water, which is then diffused through the electrolyte membrane. That is, the fuel cells have a mechanism such that the electricity is obtained when the oxygen and the fuel-derived hydrogen are fed with formation of water.
As the cathodes for use in the above fuel cells, development is underway for those composed of porous substrates on which catalyst layers (membranes) of metal components such as Pt, Pt—Ni, Pt—Co and Pt—Cu are sputtered. The anodes now studied include those composed of porous substrates on which catalyst layers (membranes) of metal components such as Pt—Ru, Pt—Fe, Pt—Ni, Pt—Co and Pt—Mo are sputtered.
However, the sputtered catalyst layers often have a problem that the fine metal particles have non-uniform and large particle diameters and consequently provide a lower surface area, resulting in insufficient activity. Moreover, the sputtering devices are expensive to cause economic difficulties.
There are other known electrodes that are composed of porous carbon materials such as carbon paper and carbon cloth on which fine platinum particles or the like are supported. Such electrodes are obtained by adhering metal salts or metal hydroxides on carbon paper and heat-treating them under a reducing atmosphere.
However, the above method has a problem in that the fine metal particles aggregate or grow during the heat treatment, so that the particle diameters become nonuniform and are difficult to control within the desired range. In addition, the catalytic activity will deteriorate with time.
The present inventors examined an enablement of particulate carrier on which metal as a catalyst were supported instead of sheet or paper carriers. It is difficult to form a thin film of the fine metal particles uniformly on the carrier particle surfaces.